Passive infrared detectors receive infrared energy from a zone of interest and produce an electrical signal representative of the differential temperature between the background ambient temperature and the temperature of a moving object or target. In one common application, passive infrared detectors are employed in security systems as intrusion detectors. In such applications, the electrical output signal from the passive infrared detector is compared against predetermined threshold limits and if the signal is outside the limits an alarm indication is provided.
The object temperature of a typical clothed human target is approximately 85.degree. F. Since a passive infrared detector senses variations in received infrared energy it is more difficult to detect a human target at 85.degree. F. then above or below this temperature. Accordingly, to achieve reliable detection of targets at 85.degree. F. or therearound it is desirable that a passive infrared detection system have a greater sensitivity at this temperature. One passive infrared detector known in the art and available as a product from the assignee of the present invention incorporates single slope temperature compensation. While such compensation provides improved immunity from false alarms at temperatures below 85.degree. F., single slope compensation provides no benefit above this temperature.